High-energy nuclear beams used in particle therapy, physical experiments and the like are produced using an accelerator. Examples of an accelerator to provide beams with kinetic energy of around 200 MeV per nucleon include a cyclotron disclosed in Patent Literature 1 and Patent Literature 2, a synchrotron disclosed in Patent Literature 3 and a variable energy accelerator disclosed in Patent Literature 4. A feature of the cyclotron is that a beam orbiting in a static magnet field is accelerated with a radiofrequency electric field, in which with increasing acceleration, the beam increases a curvature radius of its orbit, then moves to an outer orbit, and then reaching the highest energy, followed by beam extraction. Because of this, the energy of the extracted beam is fixed. The synchrotron varies, with time, the magnetic field of an electromagnet for bending the beam and the frequency of the radiofrequency electric field for acceleration in order for the beam to move in a certain orbit. Therefore, the beam is able to be extracted before the design maximum energy is reached, and also the energy to be extracted is controllable. The variable energy accelerator has a feature that, as in the case of the cyclotron, while the beam orbiting in the magnetic field is accelerated with a radiofrequency electric field, the beam orbit becomes more eccentric in one direction with increasing acceleration.